CA2237062A1

CA2237062A1 - Nonwoven web dielectric electrects with surface treatments

Info

Publication number: CA2237062A1
Application number: CA002237062A
Authority: CA
Inventors: Bernard Cohen; Lamar Heath Gipson; Joel Brostin
Original assignee: Individual
Current assignee: Kimberly Clark Worldwide Inc
Priority date: 1995-11-28
Filing date: 1996-11-21
Publication date: 1997-06-19
Also published as: SK71998A3; WO1997021364A2; DE19681669T1; GB2322874B; MX9804241A; GB2322874A; AU708641B2; ZA969816B; GB9811533D0; AU1162397A; US5834384A; WO1997021364A3

Abstract

A nonwoven web having improved particulate barrier properties is provided. A
surface treatment having a breakdown voltage no greater than 13 KV direct current is present on the nonwoven web. The particulate barrier properties are improved by subjecting said surface treatment treated nonwoven web to corona discharge.

Description

CA 02237062 1998-0~-26 W O 97/21364 PCT~US96/18772 NONWOVEN WEB DIELEC~RIC ELECTRECTS WITH SURFACE TREATMENTS

FIELD OF THE INVENTION

The pre5ent invention relates to fabrics useful for forming protective garments- More particularly, the present invention relates to nonwoven webs and surface coatings ~or such nonwoven webs.

BACKGROUN~ OF THE INVENTION
There are many types of limited use or disposable protective garments designed to provide barrier properties. Protective garments should be resistant to penetration by both liquids and~or particles.
For a variety of reasons, it is undesirable for liquids and pathogens which may be carried by liquids to pass through the garment to contact persons working in an environment where pathogens are present.
Similarly, it is highly desirable to isolate persons from harmful substances which may be present in a work place or accident site. To increase the likelihood that the protective garment is correctly worn thereby reducing the chance of exposure, workers would benefit ~rom wearing a protective garment that is relatively impervious to liquids and/or particles and durable but which is still comfortable so it does not reduce the worker's performance. After use, it is usually quite costly to decontaminate a protective garment that has been exposed to a harmful or hazardous substance. Thus, it is important that a protective garment be cost effective so as to be disposable.
One type of protective garment is disposable protective coveralls.
Coveralls can be used to effectively isolate a wearer from a harmful environment in ways that open or cloak style protective garments such as drapes, gowns and the like are unable to do. Accordingly, coveralls have many applications where isolation of a wearer is desirable.
Disposable protective garments also include disposable surgical garments such as disposable surgical gowns and drapes. As is generally known, surgical gowns and drapes are designed to greatly reduce, if not prevent, the transmission through the surgical garment of liquids and biological contaminates which may become entrained therein. In surgical CA 02237062 1998-0~-26 W O 97/21364 PCT~US96/18772 procedure environments, such liquid sources include the gown wearer's perspiration, patient liquids such as blood, salvia, perspiration and ~life support liquids such as plasma and saline.
~ Many surgical garments were originally made of cotton or linen and were sterilized prior to their use in the operating room. These surgical garments, however, permitted transmisRion therethrough or "strike-; through" of many of the liquids encountered in surgical procedures.These surgical garments were undesirable, if not unsatisfactory, because such "strike through" established a direct path for transmission of bacteria and other contaminates to and from the wearer o~ the surgical garment. Furthermore, the garments were costly, and, of course, laundering and sterili~ation procedures were required before reuse.
Disposable surgical garments have largely replaced linen surgical gowns. Because many surgical procedures require generally a high degree lS of liquid repellency to prevent strike-through, disposable surgical~garments for use under these conditions are, for the most part, made entirely from liquid repellent fabrics.
Therefore, generally speaking, it is desirable that disposable protective garments be made from fabrics that are relatively impervious to liquids and/or particulates. These barrier-type fabrics must also be .suited for the manufacture of protective apparel at such low cost that ==make discarding the garments after only a single use economical.
Examples of disposable protective garments which are generally ~ manufactured from nonwoven web laminates in order to assure that they --are cost effectively disposable are coveralls, surgical gowns and surgical drapes sold by the Kimberly-Clark Corporation. Many of the disposable protective garments sold by Kimberly-Clark Corporation are manufactured from a three layer nonwoven web laminate. The two outer layers are formed from spunbond polypropylene-based fibers and the inner ~~layer is formed from meltblown polypropylene-based fibers. The outer layers of spunbond provide tough, durable and abrasion resistant surfaces. The inner layer is not only water repellent but acts as a ~breathable filter barrier allowing air and moisture vapor to pass -~through the bulk of the fabric while filtering out many harmful 3S =particles.
In some instances, the material forming protective garments may include a film layer or a film laminate. While forming protective :garments from a fi}m may improve particle barrier properties of the ~protective garment, such film or film-laminated materials may also inhibit or prevent the passage of air and moisture vapor therethrough.

_ CA 02237062 l998-0~-26 W O 97~1364 PCT~US96/18772 Generally, protective garments formed from materials which do not allow sufficient passage of air and moisture vapor therethrough become uncomfortable to wear correctly for extended periods of time.
Thus, while in some instances, film or film-laminated materials may provide improved particulate barrier properties as compared to nonwoven-laminated fabrics, nonwoven-laminated fabrics generally provide greater wearer comfort. Therefore, a need exists for inexpensive disposable protective garments, and, more particularly, inexpensive disposable protective garments formed from a nonwoven fabric which provide improved particulate barrier properties while also being breathable and thus comfortable to wear correctly for extended periods of time.

SUMMA~Y OF TH~ INVENTION

The present invention provides a nonwoven web having improved particulate barrier properties. In one embodiment, the nonwoven web may include includes at least one layer formed from fibers subjected to corona discharge. The fibers subjected to corona discharge may include a surface treatment having a breakdown voltage no greater than 13 thousand volts (KV3 of direct current (DC) and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The nonwoven web may also include fibers formed from a blend of polypropylene and polybutylene. Desirably, the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend. Another surface treatment having a breakdown voltage greater than 13 KV DC may be present on the fibers subjected to corona discharge or on fibers not subjected to corona discharge or both.
In another embodiment, the nonwoven web may include at least one layer formed from spunbond fibers and at least one layer formed from meltblown fibers. The fibers of at least one of the layers may be ~ubjected to corona discharge and include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The nonwoven web may also include ~ibers formed from a blend of polypropylene and polybutylene. Desirably, the polybutylene is present in the blend in a range from 0.5 to ZO percent weight of the blend. Another surface treatment having a breakdown voltage greater than 13 KV DC may be present on the fibers subjected to CA 02237062 1998-0~-26 W O 97/21364 PCT~US96/18772 corona discharge or on fibers not subjected to corona discharge or both.

~ In another embodiment, the nonwoven web may include at least two layers formed from spunbond fibers and at least one layer formed from meltblown fibers. The layer formed from meltblown fibers is positioned between the two layers formed from spunbond fibers. The fibers of at least one of the layers may be subjected to corona discharge and include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 ~V DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The nonwoven web may also include fibers formed from a blend o~ polypropylene and polybutylene. Desirably, the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend. Another surface treatment having a breakdown voltage greater than 13 KV DC may be present on the fibers subjected to corona discharge or on fibers not subjected to corona discharge or both.
In another embodiment, the nonwoven web may include at least two layers formed from spunbond fibers and at least one layer formed from meltblown fibers wherein the layer formed from meltblown fibers is between the two layers formed from spunbond fibers, and wherein the fibers forming at least one of the layers are subjected to corona discharge. At least one of the layers formed from spunbond fibers may include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The layer formed from meltblown fibers includes a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown _voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The meltblown layer may further be formed from fibers which are formed from a blend of polypropylene and polybutylene, and more particularly, the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend. Another surface treatment having a breakdown voltage greater than 13 KV DC may be present on the fibers subjected to corona discharge or on fibers not subiected to corona discharge or both.
In another embodiment, the nonwoven web includes at least two layers formed from spunbond fibers and at least one layer formed from meltblown CA 02237062 1998-0~-26 W O 97/21364 PCTrUS96/18772 fibers wherein the layer formed from meltblown fibers is between the two layers formed from spunbond fibers. The fibers forming at lea3t one of the layers includes a surface treatment having a breakdown voltage no greater 13 KV DC, and wherein fibers forming another layer includes another surface treatment having a breakdown voltage greater than 13 KV
DC. Each layer formed from fibers which includes a surface treatment is subjected to corona discharge. The spunbond fiber3 of one of the layers may include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC
and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The spunbond fibers of another layer include a surface treatment having a breakdown voltage greater than 13 KV DC. The layer formed from meltblown fibers may include a surface treatment having a breakdown voltage either no greater than 13 KV DC or greater than 13 KV DC or both.

DETAIL~D DESCRIPTION OF THE INVENTION

As used herein, the term "dielectric" means, according to McGraw-Hill Encyclopedia of Science & Technology, 7th Edition, Copyright 1992, a material, such as a polymer, which is an electrical insulator or which an electric field can be sustained with a minimum dissipation of power.
A solid material is a dielectric if its valence band is full and is separated from the conduction band by at least 3 eV.
As used herein, the term "breakdown voltage" means that voltage at which electric failure occurs when a potential difference is applied to an electrically insulating material. The breakdown voltage reported for the various materials tested was determined by the ASTM test method for dielectric breakdown voltage (D 877-87).
As used herein, the term "electret" means a dielectric body possessing permanent or semipermanent electric poles of opposite sign.
As used herein, the term "surface treatment" means a material, for example a surfactant, which is present on the surface of another material, for example a shaped polymer such as a nonwoven. The surface treatment may be topically applied to the shaped polymer or may be added to a molten or semi-molten polymer. Methods of topical application include, for example, spraying, dipping or otherwise coating the shaped polymer with the surface treatment. Surface treatments which are added to a molten or semi-molten polymer may be referred to as "internal CA 02237062 l99X-0~-26 W O 97/21364 PCT~US96/18772 additives". Internal additives suitable for use in the present invention are generally non-toxic and have a low volatility. Desirably, these internal additives should be thermally stable at temperatures up to 300~
C, and su~ficiently soluble in the molten or semi-molten polymer and should also sufficiently phase separate such that said additive migrates from the bulk of the shaped polymer towards a surface thereof as the shaped polymer cools.
As used herein, the terms "necking", "neck stretching" or "necked stretched" interchangeably refer to a method of elongating a fabric, generally in the machine direction, to reduce its width in a controlled manner to a desired amount. The controlled stretching may take place ~under cool, room temperature or greater temperatures and is limited to an increase in overall dimension in the direction being stretched up to the elongation required to break the fabric, which in many cases is =about 1.2 to 1.4 times the original unstretched dimension. When relaxed, the web retracts toward its original dimensions. Such a process is disclosed, for example, in U.S. Patent no. 4,443,513 to Meitner and Notheis and in U.S. Patent no.s 4,965,122, 5,226,992 and 5,336,545 to Morman which are all therein incorporated by reference.
As used herein the terms "neck sof~ening" or "necked softened" mean neck stretching carried out without the addition of heat to the material as it is stretched, i.e., at ambient temperature. In neck stretching or softening, a fabric is referred to, for example, as being stretched by 20~.
2S As used herein, the term "nonwoven web" refers to a web that has a structure of individual fibers or filaments which are interlaid, but not in an identifiable repeating manner.
As used herein the term "spunbond fibers" refers to fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine, usually circular capillaries of a spinnerette with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Patent no. 4,340,563 to Appel et al., and U.S.
Patent no. 3,692,618 to Dorschner et al., U.S. Patent no. 3,802,817 to Matsuki et al., U.S. Patent nos. 3,338,992 and 3,341,394 to Kinney, U.S.
Patent nos. 3,502,763 and 3,909,009 to Levy, and ~.S. Patent no.
3,542,615 to Dobo et al which are all herein incorporated by reference.
Spunbond fibers are generally continuous and in some instances have an average diameter larger than 7 microns.
As used herein the term "meltblown fibers" means fibers formed by extruding a molten thermoplastic material through a plurality of fine, CA 02237062 l998-0~-26 W O 97/21364 PCT~US96/18772 usually circular, die capillaries as molten threads or filaments into a high velocity, usually heated gas (e.g. air~ stream which attenuates the filaments of molten thermoplastic material to reduce their diameter.
Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. Meltblowing is described, for example, in U.S. Patent no. 3,849,241 to Buntin, U.S. Patent no.
4,307,143 to Meitner et al., and U.S. Patent 4,707,398 to Wisneski et al which are all herein incorporated by re~erence. In some instances, meltblown fibers may generally have an average diameter smaller than lO
microns.
Polymers, and particularly polyole~ins polymers, are well suited ~or the formation of fibers or ~ilaments used in forming nonwoven webs which are u~eful in the practice of the present invention. Nonwoven webs can be made from a variety of processes including, but not limited to, air laying processes, wet laid processes, hydroentangling processes, spunbonding, meltblowing, staple fiber carding and bonding, and solution spinning.
The present invention provides a nonwoven web which may include at least one layer formed from fibers subjected to corona discharge. The nonwoven web may be formed from meltblown fibers or spunbond fibers or both. The fibers subjected to corona discharge may include a surface treatment having a breakdown voltage no greater than 13 thousand volts (KV) of direct current (DC) and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV
DC and most desirably a breakdown voltage of between 1 KV DC and 5 XV
DC. The nonwoven web may also include fibers formed from a blend of polypropylene and polybutylene. Desirably, the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend.
Another surface treatment having a breakdown voltage greater than 13 KV
DC may be present on the fibers subjected to corona discharge or on fibers not subjected to corona discharge or both.
In another embodiment, the nonwoven web may include at least one layer formed from spunbond fibers and at least one layer formed from meltblown fibers. The fibers of at least one of the layers, and desirably the layer formed from meltblown fibers, may be subjected to corona discharge and include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC

CA 02237062 1998-0~-26 W O 97/2136~ PCT~US96/18772 and 5 KV DC. The nonwoven web may also include fibers, and desirably the meltblown fibers, formed from a blend of polypropylene and polybutylene.
Desirably, the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend. Another surface treatment having a breakdown voltage greater than 13 KV DC may be present on the fibers subjected to corona discharge or on fibers not subjected to corona discharge or both.
In another embodiment, the nonwoven web may include at least two layers formed from spunbond fibers and at least one layer ~ormed from meltblown fibers. The layer formed from meltblown fibers may be positioned between the two layers formed from spunbond fibers. The fibers of at least one of the Layers, and desirably the layer formed from meltblown fibers, may be subjected to corona discharge and include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The nonwoven web may also include fibers, and desirably meltblown fibers, formed from a blend of polypropylene and polybutylene. Desirably, the polybutylene is present in the blend in a range from O.5 to 20 percent weight of the blend. Another surface treatment having a breakdown voltage greater than 13 KV DC may be present on the fibers subjected to corona discharge or on fibers not subjected to corona discharge or both.
In another embodiment, the nonwoven web may include at least two layers formed from spunbond fibers and at least one layer formed from meltblown fibers wherein the layer formed ~rom meltblown fibers may be positioned between the two layers formed from spunbond fibers, and wherein the fibers forming at least one of the layers are subjected to corona discharge. At least one of the layers formed from spunbond fibers may include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC
and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and 5 KV DC. The layer formed from meltblown fibers includes a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than S KV DC and most desirably a breakdown voltage of between 1 KV DC and S KV DC. The meltblown layer may further be formed from fibers which are formed from a blend of polypropylene and polybutylene, and more particularly, the polybutylene is present in the CA 02237062 1998-0~-26 W O 97~1364 PCTnJS96/18772 blend in a range from 0.5 to 2~ percent weight of the blend. Another surface treatment having a breakdown voltage greater than 13 KV DC may be present on the fibers subjected to corona discharge or on fibers not subjected to corona discharge or both.
In another embodiment, the nonwoven web includes at least two layers formed from spunbond fibers and at least one layer formed from meltblown fibers wherein the layer formed from meltblown fibers may be positioned between the two layers formed from spunbond fibers. The fibers forming at least one of the layers includes a surface treatment having a breakdown voltage no greater 13 KV DC, and wherein fibers forming another layer includes another surface treatment having a breakdown voltage greater than 13 KV DC. Each layer formed from fibers which includes a surface treatment is sub~ected to corona discharge. The spunbond fibers of one of the layers may include a surface treatment having a breakdown voltage no greater than 13 KV DC, and desirably a breakdown voltage no greater than 8 KV DC and more desirably a breakdown voltage no greater than 5 KV DC and most desirably a breakdown voltage of between 1 KV DC and S KV DC. The spunbond fibers of the other layer may include a surface treatment having a breakdown voltage greater than 13 KV DC. The layer formed from meltblown fibers may include a surface treatment having a breakdown voltage either no greater than 13 KV DC or greater than 13 KV DC or both.
As described in greater detail below, the entire thickness of the nonwoven web laminate may be subjected to corona discharge.
Alternatively, individual nonwoven layers which, when combined, form the nonwoven web laminate may be separately subjected to corona discharge.
When the entire thickness of the nonwoven web laminate is subjected to corona discharge, the fibers forming at least one of the nonwoven layers are desirably formed from a variety of dielectric polymers including, but not limited to, polyesters, polyolefins, nylon and copolymer of these materials. ~he fibers forming the other nonwoven layers may be formed from a variety of non-dielectric polymers, including, but not limited to, cellulose, glass, wool and protein polymers.
When one or more individual nonwoven layers are separately subjected to corona discharge, the fibers forming these nonwoven layers are desirably formed from the above described dielectric polymers. Those individual nonwoven layers which are not subjected to corona discharge may be formed from the above described non-dielectric polymers.
It has been found that nonwoven webs formed from thermoplastic based fibers and particularly polyolefin-based fibers are particularly well-CA 02237062 1998-0~-26 W O 97/21364 PCT~US96/18772 suited for the above applications. Examples of such fibers include spunbond fibers and meltblown fibers. Examples of such nonwoven webs formed from such fibers are the polypropylene nonwoven webs produced by the Assignee of record, Kimberly-Clark Corporation.
As previously described above, one embodiment of the present invention may include a nonwoven web laminate. For example, the nonwoven web laminate may include at least one layer formed from spunbond fibers and another layer formed from meltblown fibers, such as a spunbond/meltblown (StM) nonwoven web laminate. In another embodiment, the nonwoven web laminate may include at least one layer formed from meltblown fibers which is positioned between two layers formed from spunbond fibers, such as a spunbondJmeltblown~spunbond (S/M/S) nonwoven web laminate. Examples of these nonwoven web laminates are disclosed in U.S. Patent no. 4,041,203 to Brock et al., U.S. Patent 15 ~ no. 5,169,706 to Collier, et al, and U.S. Patent no. 4,374,888 to Bornslaeger which are all herein incorporated by reference. More particularly, the spunbond fibers may be formed from polypropylene.
Suitable polypropylenes for the spunbond layers are commercially available as PD-9355 from the Exxon Chemical Company of Baytown, Texas.
More particularly, the meltblown fibers may be formed from polyolefin polymers, and more particularly a blend of polypropylene and polybutylene. Examples of such meltblown fibers are contained in U.S.
Patents 5,165,979 and 5,204,174 which are incorporated herein by reference. Still more particularly, the meltblown fibers may be formed from a blend of polypropylene and polybutylene wherein the polybutylene is pre~ent in the blend in a range from 0.5 to 20 weight percent of the blend. one such suitable polypropylene is designated 3746-G from the Exxon Chemical Co., Baytown, Texas. One such suitable polybutylene is available as DP-8911 from the Shell Chemical Company of Houston, Texas.
The meltblown fibers may also contain a polypropylene modified according to U.S. patent 5,213,881 which is incorporated herein by reference.
The S/M/S nonwoven web laminate may be made by sequentially depositing onto a moving forming belt first a spunbond fabric layer, =then a meltblown fabric layer on top to the first spunbond fabric and last another spunbond fabric layer on top of the meltblown fabric layer and then bonding the laminate in a manner described below.
Alternatively, the layers may be made individually, collected in rolls, and combined in a separate bonding step. Such S/M/S nonwoven web laminates usually have an average basis weight of from about 0.1 to 12 -CA 02237062 1998-0~-26 W O 97/21364 PCTrUS96/18772 ounces per square yard (osy) (3 to 400 grams per square meter (gsm)), or more particularly from about 0.75 to about 5 osy (25 to 170 gsm) and still more particularly from about 0.75 to about 3 osy ~25 to lOO gsm).
Methods of subjecting nonwoven webs to corona discharge, are well known by those skilled in the art. Briefly, corona discharge is achieved by the application of sufficient direct current ~DC) voltage to an electric field initiating structure (EFIS) in the proximity of an electric field receiving structure (EFRS) . The voltage should be sufficiently high such that ions are generated at the EFIS and flow from the EFIS to the EFRS. Both the EFIS and the EFRS are desirably formed from conductive materials. Suitable conductive materials include copper, tungsten, stainless steel and aluminum.
One particular technique of subjecting nonwoven webs to corona discharge is the technique disclosed in U.S. Patent No. 5,401,446 which is assigned to the University of Tennessee, and is herein incorporated by reference. This technique involves subjecting the nonwoven web to a pair of electrical fields wherein the electrical fields have opposite polarities. Each electrical field forms a corona discharge.
In those instances where the nonwoven web is a nonwoven web laminate, the entire thickness of the nonwoven web laminate may be subjected to corona discharge. In other instances, one or more of the individual layers which form the nonwoven web laminate or the fibers forming such individual layers may be separately subjected to corona discharge and then combined with other layers in a juxtaposed relationship to form the nonwoven web laminate. In some instances, the electric charge on the surface of the nonwoven web laminate prior to corona discharge may be substantially the same as the electric charge on the surface of the corona discharge treated web. In other words, the surface of the nonwoven web laminate may not generally exhibit a higher electric charge after subjecting the web to corona discharge than the electric charge present on the surface of the web before subjecting it to corona discharge.
Nonwoven web laminates may be generally bonded in some manner as they aLe produced in order to give them sufficient structural integrity to withstand the rigors of further processing into a finished product.
Bonding can be accomplished in a number of ways such as hydroentanglement, needling, ultrasonic bonding, adhesive bonding and thermal bonding.

CA 02237062 1998-0~-26 W O 97/21364 PCTnUS96/18772 UltrasoniC bonding is performed, for example, by passing the nonwoven web laminate between a sonic horn and anvil roll as illustrated in U.S. Patent 4,374,888 to Bornslaeger.
Thermal bonding of a nonwoven web laminate may be accomplished by passing the same between the rolls o~ a calendering machine. At least one of the rollers of the calender is heated and at least one of the rollers, not necessarily the same one as the heated one, has a pattern which is imprinted upon the laminate as it passes between the rollers.
As the fabric passes between the rollers it is subjected to pressure as - well as heat. The combination of heat and pressure applied in a particular pattern results in the creation o~ fused bond areas in the nonwoven web laminate where the bonds thereon correspond to the pattern of bond points on the calender roll.
Various patterns for calender rolls have been developed. One example is the Hansen-Pennings pattern with between about l0 to 25~ bond area with about l00 to 5Q0 bonds/square inch as taught in U.S. Patent 3,855,046 to Hansen and Pennings. Another common pattern is a diamond pattern with repeating and slightly offset diamonds.
The exact calender temperature and pressure for bonding the nonwoven web laminate depend on the thermoplastic(s) from which the nonwoven web is made. Generally for nonwoven web laminates formed from polyolefins, desirable temperatures are between 150 and 350~F (66 and 177~C) and the pressure is between 300 and l000 pounds per lineal inch. More particularly, for polypropylene, the desirable temperature~ are between 270 and 320~F (132 and 160~C) and the pressure is between 400 and 800 pounds per lineal inch.
In those instances where the nonwoven web is used in or around flammable materials and static discharge is a concern, the nonwoven web may be treated with any number of antistatic materials. In these instances, the antistatic material may be applied to the nonwoven by any number of techniques including, but not limited to dipping the nonwoven into a solution containing the antistatic material or by spraying the nonwoven with a solution containing the antistatic material. In some instances the antistatic material may be applied to both the external surfaces of the nonwoven and/or the bulk of the nonwoven. In other instances, the antistatic material may be applied to portions of the nonwoven, such as a selected surface or surfaces thereof.
Of particular usefulness is the antistat or antistatic material known as ZELEC~, an alcohol phosphate salt product o~ the Du Pont Corporation.
-CA 02237062 1998-0~-26 W O 97/21364 PCT~US96/18772 The nonwoven web may be treated with the antistatic material either before or a~ter subjecting the web to charging. Furthermore, some or all of the material layers may be treated with the antistatic material.
In those instances where only some of the material layers are treated r 5 with antistatic material, the non-treated layer or layers may be subjected to charging prior to or after combining with the antistatic treated layer or layers.
Additionally, in those instances where the nonwoven web is used around alcohol, the nonwoven web may be treated with an alcohol repellent material. In these instances, the alcohol repellent material may be applied to the nonwoven by any number o~ techniques including, but not limited to dipping or by spraying the nonwoven web with a solution containing the alcohol repellent material. In some instances the alcohol repellent material may be applied to both the external surfaces of the nonwoven and the bulk of the nonwoven. In other instances, the alcohol repellent material may be applied to portions of the nonwoven, such as a selected surface or surfaces thereof.
Of particular usefulness are the alcohol repellent materials formed from fluorinated urethane derivatives, an example of which includes FX-1801. FX-1801, formerly called L-10307, is available from the 3M Company of St. ~aul, Minnesota. FX-1801 has a melting point of about 130 to 138~C. FX-1801 may be added to either the spunbond and/or meltblown layer at an amount of about 0.1 to about 2.0 weight percent or more particularly between about 0.2S and 1.0 weight percent. FX-1801 may be 2S topically applied or may be internally applied by adding the FX-1801 to the fiber forming polymer prior to fiber formation.
Generally, internal additives, such as the alcohol repellent additive FX-1801, suitable ~or use in the present invention should be non-toxic and have a low volatility. Additionally, the internal additive should be thermally stable at temperatures up to 300~ C, and sufficiently soluble in the molten or semi-molten ~iber forming polymer. The internal additive should also sufficiently phase separate such that said additive migrates from the bulk of the polymer fiber towards the surface o~ the polymer fiber as the fiber cools without requiring the addition of heat.
The layers of the fabric of this invention may also contain fire retardants for increased resistance to fire, pigments to give each layer the same or distinct colors, and/or chemicals such as hindered amines to provide enhanced ultraviolet light resistance. Fire retardants and pigments for spunbond and meltblown thermoplastic polymers are known in W O 97/21364 PCT~US96/18772 the art and may be internal additives. A pigment, if used, is generally present in an amount less than 5 weight percent of the layer.

To demonstrate the attributes of the present invention, several .. surface treatments were comb}ned with nonwoven webs of various average basis weights and polymer blends as listed in TABL~ I.

CA 02237062 1998-0~-26 W O 97/21364 PCTrUS96/18772 SURFACE
TREATMENT AMOUNT
INDUSTRIAL CHEMICAL APPLIED TO TYPE OF
5 DESIGNATION DESCRIPTION SURFAC~ NONWOVEN
WEB
1. Y-12488 Polyalkyleneoxide Modi~ied 4~ and 1~ 1.5 osy M
Polydimethysiloxane Union Carbide Corporation 2. HYPERM~R Modified Polyester Surfactant 4~ 1.5 osy M
A409 98~;
Xylene 2~;
ICI America Inc.

3. FC1802 C8 Fluorinated Alkyl Alkoxylate 86-89~;
C8 Fluorinated Alkyl Sulfonamide 9-10~;
C7 Fluorinated Alkyl 2.4~ 1.5 osy M
Alkocylate 2-4~;
C7 Fluorinated Alkyl Sulfonamide 0.2 1~;
3M Corp.
25 4. FX 1801 Fluorochemical Urethane 1~ 1.6 osy S/M/S
Derivative - 100~ - 3M Corp. (contained 0.03~ ZELEC) 5. TEGOPREN Polysiloxane Polyether 4~ 1.5 osy M
5830 Copolymer - Goldschmidt Corp.
6. TRITON Octlyphenoxypoylethoxy 2~ 1.5 osy M
X102 Ethanol having 12-13 Ethylene Oxide Groups - Rohm & Haas Co.
7. ZELEC Alcohol Phosphate Salt; .03~ 2.2 osy S/M/S
Neutralized Mixed Alkyl KIMGUARD~, Phosphates - Du Pont 1.6 osy S /M/ S

8. FC808 Polymeric Fluoroalphatic Ester 2.95~ 1.8 osyS/M/S
3M Corp. KLEENGUARD~
50 9. MASIL Silicon Surfactant 2~ 1.5 osy M

10.GEMTEX Dioctyl Sodium Sulfosuccinate .3~ 1.5 osy M
SM33 Based Anionic; Finetex Corp.
S/M/S Spunbond/Meltblown/Spunbond Nonwoven Web Laminate S Spunbond Nonwoven Web M Meltblown Nonwoven Web ~ All surface treatments applied topically except as noted.
~ Applied to molten polymer. Bloomed to sur~ace oi M.

CA 02237062 1998-0~-26 4 PCT~US96/18772 Applied topically to one S layer.
For samples 1-3, 5, 6, 9 and 10, the respective surface treatments were applied to a meltblown nonwoven web having an average basis weight of about 1.5 ounce per square yard (osy). These webs were made from Himont PF105 polypropylene.
For sample 4 and a portion of the nonwoven webs utilized in sample 7, the respective sur~ace treatments were applied to a S/M/S laminate having an average basis weight of about 1.6 osy. These samples included a meltblown layer having an average basis weight of about 0.5 osy between two layers of spunbond material, each spunbond layer having an average basis weight of about 0.55 osy. The spunbond layers were made from polypropylene copolymer designated PD-9355 by Exxon chemical Co.
The meltblown layer was made from polypropylene designated 3746G from 15 ~ Exxon Chemical and polybutylene ~10 weight percent) designated DP-8911 from Shell. The samples were necked softened by 8 percent at ambient temperature. The ZELEC surface treatment was present on one of the spunbond surfaces in an amount of around 0.03~ by weight of the spunbond layer. Present in the meltblown layer of each of the above samples was FX 1801.
For the remaining portion of the nonwoven webs utilized in sample 7, the ZELEC surface treatment was applied to a S/M/S laminate having an average basis weight of about Z.2 osy. Both spunbond layers had an average basis weight of around 0.85 osy and the meltblown layer had an average basis weight of around 0.5 osy. One of the spunbond layers of this sample contained about 0.03~ by weight of the spunbond layer of ZELEC surface treatment.
For sample 8, the respective sur~ace treatment was applied to a 1.8 osy S/M/S laminate. The spunbond layers were formed from polypropylene 30 ~--resins - Exxon PD-3445 and Himont PF-301. White and dark blue pigments, Ampacet 41438 ~Ampacet Inc., N.Y.) and SCC 4402 ~Standrige Color Inc., GA.), respectively, were added to the polypropylene resins forming one of the spunbond layers. The other spunbond layer was formed from these polypropylene resins without pigments. The meltblown layer was formed from the polypropylene resin Himont PF-015 without pigments.
The meltblown layer had an average basis weight of about 0.45 osy and each spunbond layer had an average basis weight of about 0.675 osy.
The 2.95~ FC808 solution was prepared by adding 0.5~ hexanol, 2.95~
FC808 and about 96.5~ water. The FC808 solution was applied to one of the spunbond layers. FC808 is an alcohol repellent surface treatment -CA 02237062 1998-0~-26 W O 97121364 PCT~US96118772 formed from a polymeric fluoroaliphatic ester ~20~), water (80~) and traces of ethyl acetate ~400 parts/million~
A portion of each of the surface treatment treated nonwoven webs described in TABLE l, (samples l-lO) was removed and not subjected to corona discharge. The remainder of each of the surface treatment treated nonwoven web samples (l-lO) was subjected to corona discharge. The corona discharge was produced by using a Model No. P/N 25A - 120volt, SO/60 Hz reversible polarity power unit (Simco Corp,, Hatfieldr PA.), which was connected to the EFIS, and a Model No. Pl6V 120V,.25A 50/60 Hz power unit (Simco Corp., Hatfield, PA.) which was connected to the EFRS.
The EFIS was a RC-3 Charge Master charge bar (Simco. Corp.) and the EFRS was a solid, three inch diameter, aluminum roller. The corona discharge environment was generally about 71~ F and 53~ relative humidity. As described in the above U.S. Patent No. 5,401,446, two sets of EFIS/EFRS are used. The voltage applied to the first set of EFIS/EFRS
was 15 KV DC/O.O KV DC, respectively. The voltage applied to the second set of EFIS/EFRS was 25 KV DC/7.5 KV DC, respectively. The gap between the EFIS and the EFRS for each set was one inch.
The filtration efficiency for both corona treated and non-corona treated nonwoven web samples was analyzed. The particulate filtration test used to evaluate the particulate filtration properties of these nonwovens is generally known as the NaCl Filter Efficiency Test (hereinafter the "NaCl Test"). The NaCl Test was conducted on an automated filter tester, Certitest~ Model # 8110, which is available from TSI Inc., St. Paul, MN. The particulate filtration efficiency of the test fabric is reported as "~ penetration". "~ penetration" is calculated by the following formula - lOO x (downstream particles/upstream particles). The upstream particles represent the total quantity of approximately O.1 ~m NaCl aerosol particles which are introduced into the tester. The downstream particles are those particles which have been introduced into the tester and which have passed through the bulk of the test fabric. Therefore, the "~
penetration" value reported in TABLEs I-V is a percentage of the total quantity of particles introduced into a controlled air flow within the tester which pass through the bulk of the test fabric. The si~e of the test fabric was 4.5" in diameter. The air flow may be constant or varied. At about 32 liters per minute of air flow, a pressure differential of between 4 and 5 mm Water Gage develops between the atmosphere on the upstream side of the test fabric as compared to the CA 02237062 1998-0~-26 W O 97/21364 PCTrUS96/18772 atmosphere on the down stream side of the test ~abric. The filtration efficiency results for samples 1-6 and 8-10 are reported in TABLE 2.
The filtration efficiency results for sample 7, the ZELEC surface treatment treated nonwovens webs, are not reported in TABLE 2.

FILTRATION EFFICIENCY
SURFACE ~ PENETRATION 0.1 ~ NaCl TREATMENT CORONA TREATED NON-CORONA TREATED
1. Y 12488 ~1~) 66.3 70.6 1. Y 12488 t4~) 54.3 55.2 2. A409 10.0 46.0 3. FC 1802 51.0 53.7 4. ZELEC + 1801 2.57 33.2 5. 5830 57.5 57 7 6. TRITON 102 1.30 51.3 8. FC808 62.4 63.0 9. SFl9 45.5 80.9 10.GEMTEX SM33 6.30 71.2 In view of TABLE 2, it was concluded that in those instances where there existed a substantial increase in filtration efficiency of the surface treatment treated nonwoven web between the non-corona treated and the corona treated, the corona tLeated nonwoven web had formed an electret. Based upon the filtration efficiency results reported in TABLE 2, four liquid surface treatments were selected for breakdown voltage analysis. The filtration efficiency data for two of the liquid surface treatments, Y 12488 and TEGOPREN 5830, indicated generally an insubstantial difference in filtration efficiency between corona and non-corona treatment. The filtration efficiency data for the other two liquid surface treatments, TRITON 102 and SF19, indicated generally a substantial improvement in the filtration e~ficiency between corona and non-corona treatment.
The breakdown voltages for these liquid surface treatments are reported in TABLE 3. The breakdown voltage for each liquid surface treatment was determined by using a Hipot Tester, model no. Hipotronics 100, having a range of 0-25 KV DC and an accuracy of +/- 2~. The CA 02237062 1998-0~-26 electrode5 were one inch diameter brass electrodes spaced 0.100 inches apart. The electrodes were submersed in a neat quantity Of the respective liquid surface treatments. The voltage to the electrodes was increased from 0 KV DC at an approximate rate of 3 KV DC/second until breakdown occurred. The electrodes and the test vessel were thoroughly waRhed, rinsed with distilled water, and air dried before testing the next surface treatment.

BREAKDOWN VOLTAGES~
MATERIAL BREA~DOWN VOLTAGE (DC) MASIL SFl9 4.8 KV

TRITON X-102 1.8 KV

~CURRENT AT BREAKDOWN VOLTAGE VARIED FROM 3.5 milliamps (MA) TO 4.9 MA.
For the two of the liquid surface treatments, Y 12488 and TEGOPREN
5830, which indicated generally an insubstantial difference in filtration efficiency between corona and non-corona treatment, the breakdown voltages were 24 KV DC and 15 KV DC, respectively. For the two liquid surface treatments, TRITON 102 and SF19, which indicated generally a substantial improvement in the filtration efficiency between corona and non-corona treatment, the breakdown voltages were 1.8 KV DC
and 4.8 KV DC, respectively.
While the invention has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of and equivalents to these embodiments. Accordingly, the scope of the present invention should be assessed as that of the appended claims and any equivalents thereto.

Claims

What is claimed is:

1. A nonwoven web comprising:
at least one layer formed from fibers, wherein fibers subjected to corona discharge include a surface treatment having a breakdown voltage no greater than 13 KV of direct current.

2. The nonwoven web of claim 1 wherein the fibers forming the layer are formed from a blend of polypropylene and polybutylene.

3. The nonwoven web of claim 2. wherein the fibers subjected to corona discharge include another surface treatment having a breakdown voltage greater than 13 KV of direct current.

4. The nonwoven web of claim 1 wherein the breakdown voltage of the surface treatment is less than 8 KV of direct current.

5. The nonwoven web of claim 2 wherein the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend.

6. A nonwoven web comprising:
at least two layers formed from spunbond fibers and at least one layer formed from meltblown fibers, wherein the layer formed from meltblown fibers is between the two layers formed from spunbond fibers, and wherein fibers of at least one of the layers are subjected to corona discharge; and wherein fibers subjected to corona discharge include a surface treatment having a breakdown voltage no greater than 13 KV of direct current.

7. The nonwoven web of claim 6 wherein the meltblown fibers are formed from a blend of polypropylene and polybutylene.

8. The nonwoven web of claim 7 wherein the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend.

9. The nonwoven web of claim 6 wherein the average basis weight of the nonwoven web is about 1.8 ounces per square yard.

10. The nonwoven web of claim 6 wherein fibers subjected to corona discharge include another surface treatment having a breakdown voltage greater than 13 KV of direct current.

11. The nonwoven web of claim 6 wherein the meltblown fibers are subjected to corona discharge.

12. A nonwoven web comprising:
at least two layers formed from spunbond fibers and at least one layer formed from meltblown fibers, wherein the layer formed from meltblown fibers is between the two layers formed from spunbond fibers, and wherein the fibers forming at least one of the layers are subjected to corona discharge; and wherein at least one of the layers formed from spunbond fibers includes a surface treatment having a breakdown voltage no greater than 13 KV of direct current and wherein the layer formed from meltblown fibers includes a surface treatment having a breakdown voltage no greater than 13 KV of direct current.

13. The nonwoven web of claim 12 wherein the meltblown fibers are formed from a blend of polypropylene and polybutylene.

14. The nonwoven web of claim 13 wherein the polybutylene is present in the blend in a range from 0.5 to 20 percent weight of the blend.

15. The nonwoven web of claim 12 wherein the breakdown voltage of the surface treatment of the spunbond fibers is less than 8 KV of direct current.

16. The nonwoven web of claim 12 where the breakdown voltage of the surface treatment of the meltblown fibers is less than 8 KV of direct current.

17. The nonwoven web of claim 12 wherein the meltblown fibers are subjected to corona discharge.

18. A nonwoven web comprising:

at least two layers formed from spunbond fibers and at least one layer formed from meltblown fibers, wherein the layer formed from meltblown fibers is between the two layers formed from spunbond fibers, and wherein fibers forming at least one of the layers includes a surface treatment having a breakdown voltage no greater 13 KV of direct current, and wherein fibers forming another layer includes another surface treatment having a breakdown voltage greater than 13 KV of direct current; and wherein each layer formed from fibers which includes a surface treatment is subjected to corona discharge.

19. The nonwoven web of claim 18 wherein the spunbond fibers of one of the layers include a surface treatment having a breakdown voltage no greater than 13 KV direct current, and wherein the spunbond fibers of another layer include a surface treatment having a breakdown voltage greater than 13 KV direct current.

20. The nonwoven web of claim 18 wherein the meltblown fibers include at least one said surface treatment.